1. Field of the Invention
This invention relates to a composition containing hollow ceramic microspheres upon which an image can be produced by subjecting the surface thereof to a source of intense radiation, and to the process for producing an image thereon.
2. Description of the Prior Art
Prior to the present invention, a number of techniques were suggested for producing materials that could be imaged upon being subjected to a source of intense radiation, such as a laser.
In the majority of the prior art techniques, the substrate is coated with a layer of a composition containing a material that absorbs the radiant energy and becomes vaporized or is otherwise removed from the surface of the substrate. Examples of radiant energy absorbing materials include various dyes and pigments, such as carbon black, and various metals, such as aluminum. If a metallic layer is coated over a dark substrate, for example, selective removal of the metallic layer will produce a positive image. If, however, a dark pigmented layer is coated over a transparent or light substrate, selective removal of the dark layer will result in a negative image.
These techniques have been employed by manufacturers of electronic components in connection with their laser marking systems. Electronic components, such as dual inline packages (DIPs), are first coated with a laser-imageable coating using conventional printing techniques such as silk screen, spraying and offset lithographic. The coated components are then selectively subjected to laser radiation which removes the coating from the areas exposed to produce an image, such as a part number or other form of identification. After printing, many electronic components are cleaned, for example in an HCl bath. Coatings containing metallic particles are attacked by the HCl to a sufficient degree to cause severe contamination of the bath and destruction of the coating.
One of the major drawbacks of the laser-imageable coating compositions that have been used is the difficulty associated with matching the color of the coating to the color of the potting compound for the electronic component. Different manufacturers prefer to distinguish their products using different colors. If the laser-imageable coating were metallic, it would have to be coated over a dark colored surface to provide the necessary contrast to the areas in which the coating had been removed by the laser. If, on the other hand, a pigment such as carbon black were to be employed in the coating, there would not be sufficient contrast against the dark background of the potting compound.
Other laser-imageable compositions are prepared with iron oxide yellow as the pigment material. Upon exposure to laser radiation, the areas exposed turn red in color due to the formation of iron oxide red.
What would be most desirable from the standpoint of color contrast, therefore, would be a coating containing a pigment or other material which changed to a color upon exposure to radiation that was in contrast with the original color of the coating. Even if such coatings were developed, however, the original colors would still have to be matched to the color of the substrate. Thus, a number of different colors which matched the colors of the substrates and which also changed to a contrasting color upon exposure to laser radiation would have to be developed.
Many of the problems associated with these laser-imageable coating compositions had been overcome by preparing radiation-imageable compositions comprising either clay or barium sulfate or a mixture thereof with a binder material. Compositions could be prepared that were essentially transparent or translucent and which turned white in color upon exposure to intense radiation. Because the coating compositions are substantially transparent, there is no longer any need to select dyes or pigments to match the color of the coating to the color of the potting compound. The white-colored image produced in the transparent coating will provide sufficient contrast against any darker color of the component over which it is coated. As a result, the manufacturer of electronic components can always maintain the desired color of the potting compound and obtain sufficient contrast between the image and the background against which it is produced.
It has been found, however, that even coating compositions containing clay and/or barium sulfate suffer from a number of drawbacks. Because the coating compositions are often highly filled with clay and/or barium sulfate, they tend to be very viscous and difficult to handle and coat onto the electronic components. Moreover, since the particles of clay and barium sulfate are generally irregular in size and shape, the surfaces of the coatings made from the compositions containing such particles are relatively rough or coarse in character and appearance. Because of this rough surface characteristic, coatings containing particles of clay and/or barium sulfate are relatively less resistant to abrasion.